well dudes, I like your ideas. my personal favourite has always been patatin patatan, even though its probably not estilo espanol I always found prices pretty attractive, you get a meal of chips, bocadillo and a soft drink/beer for something like 750 ptas. there should be several locations and 2 I've been to are at c/goya esquina (corner) dr. esquerdo and the other at c/ibiza and c/menendez pelayo at the east side of the retiro park - just in case you should be visiting that area.... have fun, I myself will be going back in september - for some great 3 years at least :-)
suerte !